
IBM21P100BGB
IBM 133 PCI-X Bridge R1.1
ppb11_pcix_regs.fm.03
July 9, 2001
Configuration Registers
Page 61 of 131
5.2.5.4 Arbiter Mode Register
The Arbiter Mode register provides controls for the secondary bus arbitration logic on the bridge.
Address Offset
x‘50’
Access
See bit descriptions for details
Reset Value
x’0800’
When S_INT_ARB_EN# (T21) is tied low. See
Reset Value
x’0801’
When S_INT_ARB_EN# (T21) is tied high. See
A
rbit
e
r
F
airness
C
ount
er
Re
s
e
rv
ed
B
roken
Ma
st
er
T
imeout
E
nable
E
x
te
rnal
A
rbit
e
r
b
it
15 14 13 12 11 10
9
876543210
Bit(s)
Access
Field Name and Description
15:8
RW
Arbiter Fairness Counter
This is the initialization value of a counter used by the internal arbiter. It controls the number of PCI bus cycles
that the arbiter holds a device’s PCI Bus Grant active after detecting a PCI Bus Request from another device.
The counter is reloaded whenever a new PCI Bus Grant is asserted. For every new PCI Bus Grant, the counter
is armed to decrement once it detects the new fall of FRAME#. If the Arbiter Fairness Counter is set to x‘00’,the
arbiter will not remove a device’s PCI Bus Grant until the device has deasserted its PCI Bus Request.
The reset value is x‘08’.
7:2
RO
Reserved
1RW
Broken Master Timeout Enable
This bit enables the internal arbiter to count 16 PCI bus cycles waiting for FRAME# to become active when a
device’s PCI Bus Grant is active and the PCI bus is idle. If the Broken Master Timeout expires the PCI Bus
Grant for the device is deasserted.
0
Broken Master Timeout disabled
1
Broken Master Timeout enabled
The reset value is b‘0’.
0RO
External Arbiter bit
This status bit reflects whether the bridge is in internal or external arbiter mode. The value is set at reset time,
according to the polarity of the S_INT_ARB_EN# I/O pin.
0
Internal arbiter in control
1
External arbiter in control